In recent years, there has been a growing need for intermediate storage of spent nuclear fuel (which will be hereinafter referred to as “SF”) in a nuclear power plant. Further, there is a trend of shift from wet storage (storage in water) to dry storage (storage by air cooling) for intermediate storage of SF. Accordingly, SF has a higher heat value and neutron generation density than in the related art. Therefore, a boron-containing aluminum material for forming a cask or a canister which is a SF storage container is also required to have a higher boron content than ever before.
For manufacturing a boron-containing aluminum alloy, a melting/casting method has been used in the related art. The melting/casting methods include: a method in which a powdery boron is added to an aluminum alloy base metal, and it is molten and casted (which will be hereinafter referred to as “the former melting/casting method”); or a method in which fluoroborate such as KBF4 is added with a catalyst into an aluminum molten metal to generate an aluminum-boron intermediate alloy, followed by adjustment of the boron concentration for casting (which will be hereinafter referred to as “the latter melting/casting method”. The ingot casted in this manner is formed into a sheet material by rolling treatment, extrusion treatment, or the like.
With the former melting/casting method described above, there are formed various boron compounds crystallized into the aluminum-boron alloy, resulting in degradation of machinability. Further, a difference in specific gravity among various formed boron compounds causes the boron compounds to precipitate or (to float), resulting in ununiform distribution (i.e., segregation) of boron. This leads to the formation of sites with a lower concentration than the added boron content. As a casting, the upper limit of the actually obtainable boron concentration is about 1 mass %.
Further, the latter melting/casting method described above essentially requires boron (concentrated boron) increased in content of boron isotope with a mass number of 10 absorbing thermal neutrons present in a ratio of 18.4 mass % in natural boron (which will be hereinafter referred to as “B-10”). However, the concentrated boron is very expensive, incurring a problem in terms of cost.
Under such circumstances, the following technologies are also proposed.
There is disclosed a metal matrix composite material having a pair of metal sheets including a mixed material interposed therebetween: the mixed material is a metal matrix composite material including a metal powder and ceramic particles having a neutron absorbing function; the ceramic particles include B4C particles; the area density of B-10 included in the B4C particles is set at 40 mg/cm2 or more; and the neutron absorption rate achieved by the B4C particles is 90% or more (see Patent Literature 1).